Palladium/charcoal

Catalytic reduction of quinazolines unsubstituted in position 4 using palladium-charcoal, palladium on calcium carbonate, Raney nickel, or Adam s platinum has been used for preparing 3,4-dihydro-... 

Saturation of a carbohydrate double bond is almost always carried out by catalytic hydrogenation over a noble metal. The reaction takes place at the surface of the metal catalyst that absorbs both hydrogen and the organic molecule. The metal is often deposited onto a support, typically charcoal. Palladium is by far the most commonly used metal for catalytic hydrogenation of olefins. In special cases, more active (and more expensive) platinum and rhodium catalysts can also be used [154]. All these noble metal catalysts are deactivated by sulfur, except when sulfur is in the highest oxidation state (sulfuric and sulfonic acids/esters). The lower oxidation state sulfur compounds are almost always catalytic poisons for the metal catalyst and even minute traces may inhibit the hydrogenation very strongly [154]. Sometimes Raney nickel can... 

To do the reaction, a champagne bottle of at least 1.5 liters volume is filled with 50 grams sodium acetate (anhydrous) and 700 ml of distilled water. The pH of this solution is then made neutral (pH 7) by dripping in diluted acetic acid. This forms an acetic buffer which prevents the solution from becoming acidic when chlorephedrine hydrochloride is added to it. It also neutralizes the hydrochloric acid formed when the chlorine atom is removed from the chlorephedrine molecule. Then 40 grams of 5% palladium black on charcoal (palladium content 2 grams) is added, and finally 125 grams of chlorephedrine hydrochloride is added. 

The palladium dispersion on the activated charcoal (AC) was somewhat different when compared to that observed on the CNTs catalyst. On the activated charcoal, palladium was present in agglomerate shape instead of individual particles as observed on the CNTs, which led to a less homogeneous dispersion of the metal particles on the support. However, the average particle size estimated fi om TEM was similar to that of the palladium supported on the CNTs, i.e. 5 nm. 

Patrick and coworkers [66] developed an efficient method for synthesis of biaryls by slow addition of two equivalents of trifluoroaeetic acid to the solution of the aryltriazenes (VII) in an arene at 65-70 C. As reaction proceeds, the nitrogen is evolved from the reaction mixture and biaryls (II) are formed in fair yields. The efficacy can be judged from the selected examples presented in the Scheme 18. The yields can be further increased for 10-15% by adding a catalytic amount of iodine (5-10 mol%), 5% palladium on charcoal, palladium(II) acetate (5 mol%), or acetonitrile as a cosolvent (ca. 10-25%). 

For the work-up, it was planned to distill off the isopropanol and to extract the product with an organic solvent. From this solution, the palladium should be removed by adsorption onto a scavenger. This task turned out to be the main challenge of the project. Several solvents such as alcohols, acetone, xylene, and dichloromethane were checked in combination with different absorbents, e.g., activated charcoal, alumina, and silica. In most solvents, the solubility of the product was very poor at room temperature. The solubility was only sufficient in acetone and dichloromethane. The best results were obtained by slow filtration of a dichloromethane solution through a bed of silica and activated charcoal. Palladium was eliminated almost completely, and less than 10 ppm of palladium was fotmd in the final product. 

Preparation of 30 per cent, palladium or platinum catalysts (charcoal or asbestos carrier). 

Method A. Cool a solution of the nitrate-free dichloride, prepared from or equivalent to 5 0 g. of palladium or platinum, in 50 ml. of water and 5 ml. of concentrated hydrochloric acid in a freezing mixture, and treat it with 50 ml. of formahn (40 per cent, formaldehyde) and 11 g. of the carrier (charcoal or asbestos). Stir the mixture mechanically and add a solution of 50 g. of potassium hydroxide in 50 ml. of water, keeping the temperature below 5°. When the addition is complete, raise the temperature to 60° for 15 minutes. Wash the catalyst thoroughly by decantation with water and finally with dilute acetic acid, collect on a suction filter, and wash with hot water until free from chloride or alkali. Dry at 100° and store in a desiccator. 

Broadly speaking, the differences in effectiveness of palladium and platinum catalysts are very small the choice will generally be made on the basis of availability and current price of the two metals. Charcoal is a somewhat more efficient carrier than asbestos. 

The addition of N-bromosuccinimide (1.1equiv) to a dichlo-romethane solution containing the alkene (1 equiv) and cyana-mide (4 equiv). The solution was maintained at room temperature (3 days) and then washed with water, dried, and concentrated in vacuo. Treatment of the bromocyanamide [intermediate] with 1% palladium on charcoal in methanol (1h) led to reduction of the for-madine. Addition of base to the reaction mixture (50% aqueous KOH, reflux 6h) followed by extraction with ether gave monoamine. (Yield is 48-64% final amine from alkenes analogous to safrole)... 

Phenylpropanolamine. - With catalyst prepared as previously described from 0.5g of palladium chloride and 3g of charcoal, it was possible to reduce two portions of 9.8g of isonitrosopropio-phenone (0.06 mol), dissolved in 150 cc. of absolute alcohol containing 7. Og of hydrogen chloride, to phenylpropanolamine in from 145 - 190 minutes with yields of the isolated chloride from 9.4g to 11. Og, or 84 to 98% of the theoretical. After recrystallization from absolute alcohol the salt melted at 191°. The free base was obtained by treating an aqueous solution of the hydrochloride with alkali on cooling, the liberated amino alcohol solidified and after recrystallization from water melted at 103°."... 

More conveniently, compound (13) was directly condensed with barbituric acid (14) in acetic acid (28) or in the presence of an acid catalyst in an organic solvent (29). The same a2o dye intermediate (13) and alloxantin give riboflavin in the presence of palladium on charcoal in alcohoHc hydrochloric acid under nitrogen. This reaction may involve the reduction of the a2o group to the (9-phenylenediamine by the alloxantin, which is dehydrogenated to alloxan (see Urea) (30). 

Later, a completely different and more convenient synthesis of riboflavin and analogues was developed (34). It consists of the nitrosative cyclization of 6-(A/-D-ribityl-3,4-xyhdino)uracil (18), obtained from the condensation of A/-D-ribityl-3,4-xyhdine (11) and 6-chlorouracil (19), with excess sodium nitrite in acetic acid, or the cyclization of (18) with potassium nitrate in acetic in the presence of sulfuric acid, to give riboflavin-5-oxide (20) in high yield. Reduction with sodium dithionite gives (1). In another synthesis, 5-nitro-6-(A/-D-ribityl-3,4-xyhdino) uracil (21), prepared in situ from the condensation of 6-chloro-5-nitrouracil (22) with A/-D-ribityl-3,4-xyhdine (11), was hydrogenated over palladium on charcoal in acetic acid. The filtrate included 5-amino-6-(A/-D-ribityl-3,4-xyhdino)uracil (23) and was maintained at room temperature to precipitate (1) by autoxidation (35). These two pathways are suitable for the preparation of riboflavin analogues possessing several substituents (Fig. 4). 

Nitropyridazines are reduced catalytically either over platinum, Raney nickel or palladium-charcoal catalyst. When an N-oxide function is present, palladium-charcoal in neutral solution is used in order to obtain the corresponding amino N-oxide. On the other hand, when hydrogenation is carried out in aqueous or alcoholic hydrochloric acid and palladium-charcoal or Raney nickel are used for the reduction of the nitro group, deoxygenation of the N- oxide takes place simultaneously. Halonitropyridazines and their N- oxides are reduced, dehalogenated and deoxygenated to aminopyridazines or to aminopyridazine N- oxides under analogous conditions. 

Hydroxyaminopyridazine 1-oxides are usually formed by catalytic hydrogenation of the corresponding nitro derivatives over palladium-charcoal in methanol, provided that the reaction is stopped after absorption of two moles of hydrogen. 3-Hydroxyaminopyridazine 1-oxide and 6-amino-4-hydroxyamino-3-methoxypyridazine 1-oxide are prepared in this way, while 5-hydroxyamino-3-methylpyridazine 2-oxide and 5-hydroxyamino-6-methoxy-3-methylpyridazine 2-oxide are obtained by chemical reduction of the corresponding nitro compounds with phenylhydrazine. 

Sometimes bimolecular products are formed from reduction of nitropyridazine AT-oxides. For example, 3-acetamido-6-methoxy-5-nitropyridazine 2-oxide, on reduction over palladium-charcoal, affords two different products, depending on the reaction conditions (Scheme 49). 

Catalytic hydrogenation of 3,6-diphenylpyridazine 1,2-dioxide over palladium-charcoal affords 3,6-diphenylpyridazine 1-oxide and 3,6-diphenylpyridazine (79JOC3524). 

The best way to make pyrimidine in quantity is from 1,1,3,3-tetraethoxypropane (or other such acetal of malondialdehyde) and formamide, by either a continuous (58CB2832) or a batch process (57CB942). Other practical ways to make small amounts in the laboratory are thermal decarboxylation of pyrimidine-4,6-dicarboxylic acid (744), prepared by oxidation of 4,6-dimethylpyrimidine (59JCS525), or hydrogenolysis of 2,4-dichloropyrimidine over palladium-charcoal in the presence of magnesium oxide (53JCS1646). 

Pyrazine 1,4-dioxides are available by the direct self-condensation of 1,2-hydroxyaminooximes (70JOC2790). 1,2-Nitrooximes are obtained by the isomerization of alkene initrogen trioxide adducts, which are reduced with palladium on charcoal to the hydroxyaminooximes which undergo acid-catalyzed auto-condensation to the pyrazine 1,4-dioxides (Scheme 19). 

Aryl tetrazolyl ethers (519) are reduced by palladium on charcoal to give the arene and the tetrazolinone (520) (77AHC(2D323) this reaction is used for the removal of phenolic functionality. 

The synthesis of indazoles from their 4,5,6,7-tetrahydroderivatives (439) by means of sulfur or, better, by catalytic dehydrogenation over palladium on charcoal (67HC(22)l) can also be included here. 

B. 2,2,7,7,12,12,17,17-Octemfi<%Z-21,22,23,24-t tfstainless-steel shaking autoclave is charged with 4.0 g. (0.0092 mole) of the tetraoxaquaterene from Part A, 200 ml. of ethanol, and 400 mg. of 5% palladium on charcoal catalyst (Note 6). The autoclave is filled with hydrogen at an initial pressure of 170 atm. and heated with shaking for 4 hours at 105 . Catalyst and a white solid are removed by filtration (Note 7), the solid is dissolved in 100 ml. of warm chloroform, the solution is filtered, the chloroform is evaporated, and the white solid which is obtained is dried under reduced pressure at 60° (Note 8). The tetraoxaperhydroquaterene is obtained as a white solid, m.p. 204-209 (Note 9), in a yield of 2.85-2.97 g. (69-72%). 

Catalyst obtained from Engelhard Industries was used. The submitters used 200 mg. of Fluka 10% palladium on charcoal catalyst with 5 g. of starting material in 250 ml. of ethanol and obtained a total yield of 2.3 g. (46%), m.p. 208-211°. 

A route to phenol has been developed starting from cyclohexane, which is first oxidised to a mixture of cyclohexanol and cyclohexanone. In one process the oxidation is carried out in the liquid phase using cobalt naphthenate as catalyst. The cyclohexanone present may be converted to cyclohexanol, in this case the desired intermediate, by catalytic hydrogenation. The cyclohexanol is converted to phenol by a catalytic process using selenium or with palladium on charcoal. The hydrogen produced in this process may be used in the conversion of cyclohexanone to cyclohexanol. It also may be used in the conversion of benzene to cyclohexane in processes where benzene is used as the precursor of the cyclohexane. 

A suitable catalyst is 10% palladium-on-charcoal, such as is supplied by Baker and Company, Inc., 113 Astor Street, Newark 5, New Jersey. 

Snyder and Smith prepared diethyl acetamidomalonate in 40% yield by reduction of diethyl isonitrosomalonate in ethanol over palladium on charcoal followed by direct acetylation of diethyl aminomalonate in the filtrate with acetic anhydride. Ghosh and Dutta used zinc dust instead of palladium. A modification using Raney nickel is described by Akabori et al. Shaw and Nolan reported a 98% yield by conversion of diethyl oximino-malonate-sodium acetate complex. 

Diethyl isonitrosomalonate has been reduced catalytically, over palladium on charcoal, Raney nickel, and chemically by aluminum amalgam or hydrogen sulfide. ... 

Triphenylene has been prepared by self-condensation of cyclohexanone using sulfuric acid or polyphosphoric acid followed by dehydrogenation of the product, palladium-charcoal, or selenium by electrolytic oxidation of cycloliexanone from chlorobenzene and sodium or phenyllilhium from 2-cyclolu xyl-l-phenylcyelohexanol or... 

Normally, the hydrogenation of a readily hydrogenated double bond occurs over palladium-on-charcoal in ethanol at room temperature and atmospheric pressure. The more difficultly reduced olefins require elevated reaction temperatures and/or pressures for the reaction to proceed at a reasonable rate. The saturation of an 8(14)-double bond is virtually impossible under normal hydrogenation conditions. In order to remove unsaturation at this position it is necessary to first isomerize the double bond to the readily hydrogenated 14 position by treatment with dry hydrogen chloride in chloro-form. ° ... 

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